(A) Discussion of the Prior Art.
FIG. 10 shows the configuration of an existing optical scanner and a light beam printer using this scanner. The scanner uses polygonal mirror 31, which is in the shape of a regular polygon whose outer edges are coated with mirror surfaces 31a, 31a, etc. Polygonal mirror 31 is rotated at a fixed angular velocity by DC servomotor 32, which is controlled by driver circuit 35. Laser beam .alpha., emitted by semiconductor laser device 36, is focused by imaging lens 33 and projected onto one of the mirror surfaces 31a of polygonal mirror 31. When laser beam .alpha. is reflected off mirror surface 31a, it passes through beam scanning lens 34 and strikes the surface of, for example, a light sensitive drum 37. For a code reader, the drum 37 is replaced by a medium to be read and a focusing lens, not shown, is provided between the scanning lens 34 and read medium.
When polygonal mirror 31 is rotating at a fixed angular velocity, the angle at which laser beam .alpha. strikes mirror surface 31a will vary, and consequently the direction in which laser beam .alpha. is reflected will also vary. In this way the laser beam .alpha. can scan the surface of, for example, light-sensitive drum 37.
With this type of optical scanner, a polygonal mirror and a DC servomotor to drive its rotation are required. This has made it very difficult to reduce the size of the optical scanner, and it has placed a limit on how much size reduction is possible. Furthermore, in order to achieve precision in the width of the area scanned, the scanning speed and other scanning characteristics a high degree of accuracy is required in the dimensions of the mirror surfaces on the polygonal mirror, the angle at which each pair of surfaces meets, and so on. Accordingly, processing costs and the cost of assembly and adjustment are quite high and difficult to reduce. Another problem is that the angular range of scanning is fixed for each optical scanner, as the angle over which the laser beam scans is determined by the number of mirror surfaces. It is thus impossible to alter the angular range of scanning.
Furthermore, existing optical scanners are capable of scanning a laser beam in one direction only. A single scanner is incapable of switching between different directions of scanning or of scanning in two directions simultaneously.
(B) Summary of the Invention
In light of the faults of existing optical scanners which have been described above, the object of this invention is to provide a compact, economically priced optical scanner which is based on a new principle, and a light beam printer which employs this scanner.
In one embodiment of the invention, the optical scanner of this invention comprises a shaft capable of at least two modes of elastic deformation; a vibrating element provided on one end of the deformable shaft; a drive source for inducing oscillation in the vibrating element at a resonant frequency corresponding to each mode of elastic deformation of the shaft; a scanning element which can be caused to rotate in at least two directions by the elastic vibration of the deformable shaft, which scanning element is located on the opposite end of the deformable shaft, and is positioned in such a way that it can receive elastic vibrations in the shaft in at least one mode of deformation when oscillation is induced in the vibrating element; and a mirror surface on the scanning element.
The light beam printer of this invention comprises a source to generate a light beam; the optical scanner described in the preceding paragraph which reflects this light beam and causes it to scan; and a medium for receiving the scanning beam.
The operation of this first embodiment of the invention is briefly described as follows: Vibration of a resonant frequency corresponding to a specified mode of elastic deformation of the deformable shaft is applied to the vibrating element. The deformable shaft undergoes elastic vibration in the desired mode of elastic deformation, and the scanning element rotates in a specified direction. When a light beam strikes the mirror surface on the scanning element, the rotation of the scanning element causes the light beam reflected by the mirror surface to scan.
The deformable shaft has at least two modes of elastic deformation. When the drive frequency originating in the drive source is changed so as to alter the excitation mode, the direction in which the scanning element rotates will change. Thus, the direction in which the light beam scans can be changed. A single optical scanner, then, can be made to scan in at least two directions.
The scanning element, the deformable shaft and the vibrating element can all be made in the form of a flat plate, and a very small actuator, such as a piezoelectric or magnetostrictive vibrator, can be used as the drive source. Thus, the first embodiment optical scanner has enormous potential to be downsized. The structure of this first embodiment is uncomplicated, so the production cost as well as the assembly and adjustment cost will be low.
Furthermore, the amplitude of elastic vibration occurring in the deformable shaft (the angle of rotation of the scanning element) can be changed by causing the drive source to vary the amplitude at which the vibrating element oscillates. Thus it is possible to adjust the angle over which the light beam scans.
In a second embodiment of the invention, the optical scanner of this invention comprises a torsional spring to which torque can be applied; a scanning element attached to one end of the torsional spring, with its center of gravity being separated from the rotational axis of the torsional spring; a mirror surface on the scanning element; a vibrating element mounted on the other end of the torsional spring; and a drive unit to provide vibration to the vibrating element.
The operation of this second embodiment of the invention is briefly described as follows: The center of balance of the scanning element is distinct from that of the torsional spring, and thus the scanning element is unbalanced with respect to the center of balance of the torsional spring. When oscillation is induced in the vibrating element, torsional vibration is induced in the torsional spring, and the scanning element rotates. A light beam is directed to the mirror surface on the scanning element, and the rotation of the scanning element will cause the light beam reflected off the mirror to scan a surface.
The scanning element, the torsional spring and the vibrating element can all be made in the form of a flat plate, and a very small actuator, such as a piezoelectric or magnetostrictive actuator, can be used as the drive source. An optical scanner constructed using only these two types of materials has enormous potential to be downsized. The structure of the second embodiment optical scanner is also uncomplicated, so the production cost as well as the assembly and adjustment cost will also be low.
In addition, in the second embodiment the drive source can be used to change the amplitude at which the vibrating element oscillates. In this way the amplitude of the torsional vibration of the torsional spring (the angle of rotation of the scanning element) can be changed, and the angle over which the light beam scans can be adjusted.
The use of the first and second embodiments of optical scanners also makes it possible to produce compact economical code readers and light beam printers which operate at high speed.
The foregoing and other features and advantages of the invention will be more clearly understood from the following detailed description which is provided in connection with the accompanying drawings.